This invention relates to the manufacture of metallic tubing. It is more particularly concerned with a method for manufacturing thin-walled zinc coated electrical metallic tubing, fence post tubing, ladder railings and the like, and with the tubing so produced.
The uses of fence post tubing, ladder railings and the like are self-explanatory and are well known. Generally, such tubing is produced by the same process as described for electrical metallic tubing. Electrical metallic tubing is used for metal raceways for the installation of electrical wires and cables. Large quantities of this tubing are made of steel in size ranges from nominal 3/8 inch to nominal 4 inch diameter. The tubing is relatively thin-walled, having a wall thickness of about 0.042 inch in the smallest sizes and increasing to about 0.083 inch for nominal 4 inch tubing. Steel tubing of this type is conventionally made by forming a flat blank into a tube and welding the edges together. The smaller sizes of electrical metallic tubing are often bent in fabrication and must be able to withstand bending without cracking, rupture or collapse. Tubing of 1/2 inch nominal or trade size, for example, must be able to withstand bending into a semi-circle the inner edge of which has a radius of 31/2 inches and subjected to a hydrostatic pressure of 30-50 psi to test for seam cracks or openings.
Steel tubing of this type is commonly protected by a zinc coating. The tubing after forming and welding is conventionally hot dip galvanized, electro-galvanized or steam metallized so that the weld is coated to the same extent as the remainder of the surface. The Underwriters' Laboratories require that the galvanized coating on the exterior of the tubing meet certain thickness standards described hereinafter. These standards do not apply to the coating on the inside of the tube, but that coating must protect the tubing against corrosion. In practice, therefore, the inside surface of the formed tubing is spray or flow coated with an enamel, paint or other acceptable coating.
The coating thickness test specified by the Underwriters' Laboratories is commonly known as the Preece test and is described in detail in the Underwriters' Laboratories standards for electrical metallic tubing, UL 797. Specimens of the zinc coated steel are immersed or dipped in a copper sulfate solution of prescribed strength for sixty seconds and are then removed and washed in running water. The zinc from the specimen displaces copper from the solution, which plates out on the specimen. The copper does not adhere strongly to zinc, however, and the loosely adhering deposits are removed by washing in water, followed by wiping the specimen with cheesecloth. The procedure described is then repeated, to an end point described hereinafter. The coating thickness is determined by the number of successive dips which the coating can withstand without dissolving to the steel base. When the zinc is removed down to the iron, which also displaces copper from the solution, the copper adheres firmly to the iron and cannot be washed or rubbed off. The zinc coating of electrical metallic tubing must withstand four such immersions or dips without showing a final firm deposit of copper.
It is economically advantageous to manufacture various types of tubing, including electrical metallic tubing, with an outside coating of zinc which meets the Underwriters' Laboratories requirements above set out but with an inside coating only thick enough to prevent corrosion, and it is the principal object of this invention to provide such tubing. Another object is to provide a process of manufacturing such tubing. Other objects of this invention will appear in the course of the description thereof which follows.
It has been found that metallic tubing can be formed and welded from galvanized steel strip provided with a relatively heavy coating of zinc on the side which forms the outside of the tubing and with a relatively lighter coating of zinc on the side which forms the inside of the tubing. The zinc which is unavoidably melted or volatilized in the weld area by the heat of welding or removed by subsequent scarfing is replaced by atomization metallizing in the way hereinafter described. Tubing produced according to this process meets Underwriters' Laboratories specifications for electrical metallic tubing.
Economic considerations require that the metallizing be done in line with the continuous forming and welding operation. Previously, attempts have been made to do this with zinc, but none of these produced zinc coatings meet the Underwriters' Laboratories bend test. In atomization metallizing the coating metal is melted and atomized onto the surface to be coated. The atomization device, usually called a gun, is fed with coating metal in wire or powder form and after melting discharges the atomized coating metal onto the tubing. As the tubing is formed and welded at speeds in excess of 100 feet per minute, and the width of the area requiring metallizing is quite small, on the order of 3/16 inch, the restrictions thus imposed on metallizing are severe. In order to meet the Underwriters' Laboratories coating thickness requirements, it would appear that a substantial thickness of zinc must be deposited on the substrate, moving at the speeds above mentioned. When it is attempted to deposit a reasonably thick coating of zinc by metal spraying, particularly on a hot substrate such as a welded tube, the heat input tends to cause the deposited zinc to volatilize or sublime. The more zinc deposited on the metal the more this tendency increases, leading toward an equilibrium condition in which the deposition of more molten zinc results in the volatilization of an equal amount of zinc.
In experiments it has proved impossible, using one gun only, to continuously spray metallize with zinc the weld zone of continuously welded tubing coming from the welder so as to deposit consistently a coating which met the Underwriters' Laboratories test previously described. Processes using two zinc guns in tandem produced no better results. Adjustment of the relative amounts of zinc sprayed by each gun resulted in little improvement. Generally, these zinc coatings were non-uniform in thickness being thinnest at the weld seam, indicating need for a better substrate and heat control application at weld zone area. Experiments were also made with a process in which commercially available EC (electrical conductor grade) aluminum was sprayed first and then zinc was sprayed onto it, but the resulting product was not satisfactory.
It is also known that steel tubing can be satisfactorily atomization metallized continuously along the weld in line with the tube-forming and welding apparatus by a two-stage process in which zinc is melted and atomized onto the tubing in the second stage, in an area which includes the weld area. Such tubing made from galvanized strip meets all the requirements for fence post tubing, ladder railing and the like and all Underwriters' Laboratories standards for electrical metallic tubing.